In 1971, in the course of examination of results of tests carried out with the process of reference 1, the writer became aware of the fact that, if, in the production of porous nickel substrate type positive electrodes of nickel-cadmium battery cells, the extent of the pore loading was held to two thirds of what is customary in the case of cells of batteries of aircraft type, it became possible to recharge to 95 percent of full capacity in 2 minutes without evidence of gas evolution or cell damage, whereas equally fast charging of more fully pore loaded electrodes to the same state of charge caused early gassing and evidence of electrode damage.
Because of the fast discharge-recharge properties, and the well established long discharge-recharge cycle life of nickel-cadmium batteries, it occurred to the writer, that, with the benefit of reduction in cost that could result from large scale production, nickel-cadmium batteries of reduced pore loading might prove useful as power storage components of hybrid drive systems for road vehicles, such as taxis and within city busses which operate constantly in city traffic.
In August of 1971 the writer met with J. L. Hartman, head of the Electrochemistry Dept. of the General Motors Technical Center, at Warren, Mich., and suggested that nickel-cadmium batteries could prove to be useful as components of hybrid-drive systems for limited market vehicles, including taxis, for which the aspect of low annual production of cadmium would not stand in the way of employment.
In this meeting it came out that General Motors had already carried out studies that indicated the advantages of Ni-Cd batteries as the power battery component of hybrid drive systems of dual battery, all-electric type, but that the factor of high cost, plus limitations as to availability of cadmium, had deflected interest, as see reference 2.
The meeting resulted, however, in a telephone contact with the Air Pollution Office of the Environmental Protection Agency at Ypsilanti, Mich., which had recently sponsored a program of development aimed at producing a power battery that would deliver 55 kw for 10 seconds, and led to the writer being furnished a copy of reference 3, in which mention is made of the potential value of Ni-Cd batteries as power sources for use in engine-electric hybrid drive systems of taxis and busses.
In September 1972 the writer contacted Lawrence Foote of the Ford Motor Scientific Laboratory, who, for some years, had been working on development of electric vehicles, and undertook to develop interest on his part in hybrid drive systems that would employ a nickel-cadmium battery, but that, due to limited availability of cadmium, would require to be used to power limited market vehicles.
Sometime after this conversation, the Ford Motor group that was assigned to work in the area of electric vehicles developed interest in engine-electric type hybrid drives, as a way to protect the limited volume market for large high priced personal use vehicles for which there is especial need to reduce fuel usage per mile.
In due course, that group obtained authority to carry out full-scale dynamometer tests of a hybrid drive system of engine-electric type, and decided, also, to use in these tests, nickel-cadmium cells, that would be produced by the Marathon Battery Co., and that, per the writer's recommendations, would be modified in manufacture, to exclude use of cellophane as a component of the separator, and to incorporate a centrally located plastic spacer that would reduce effective cell thickness, and thereby improve ability to cool.
The results of the tests, which were published in reference 4 were to the effect that, in simulated traffic conditions, a hybrid drive system for a large vehicle utilizing an engine of somewhat less than usual size, and in which it was a feature of the drive system that the engine would stop when the vehicle came to rest, could render it possible to effect a 50 percent reduction in fuel usage per mile of travel.
However, because of the cost and weight of the battery that was employed in the tests, interest developed in investigating a drive system employing a full size engine and a smaller battery which would be called on merely to participate in the acceleration of the vehicle from standstill to around 15 miles per hour, while at the same time starting the engine, and that, in normal operation, would be fully recharged via regenerative braking.
The calculated advantages of this new strategy, in which duration of discharge would typically be held to only 3 seconds are set forth in reference 5, where it was referred to as Ford B strategy, comprised a fuel economy superior to that found with use of the cycle that had been tested and reported on in reference 4, which was identified in reference 5 as Ford A strategy.
Of special interest to the writer was the fact that, in Table IV of reference 5 the overall efficiency of a taxi in conventional drive is given as 0.074, while for hybrid drive with use of the Ford B strategy it is given as 0.199, which implies that a hybrid drive of Ford B strategy type would use only (0.074/0.199).times.100=37.3 percent as much fuel as would a conventional taxi and hence would improve fuel economy by 62.7 percent.
Following the tests described in reference 4, the Ford Scientific Laboratory decided to determine, by test, how well lead-acid batteries would perform with use of the new strategy.
However, after the writer brought out that,
(a) in fast cycling tests of small experimentally produced cells, which were first discharged for 3 secs. and thereafter at once recharged for 3 secs. at a rate that would correspond to use of a 160 pound vehicle battery, a cycle life in excess of 400,000 had been achieved, with absence of need to vent internally generated gas, PA1 (b) increase in current density by a factor of two, which would correspond to a battery of 80 pounds weight neither caused gassing nor reduced discharge-recharge efficiency below 75 percent, PA1 (c) in the tests a flooded cell had been used in which the active material content of the pores of the positive electrodes per unit of area was 56 percent of the figure that typically applies to a typical commercial aircraft battery, PA1 (d) it might be found possible to reduce the cadmium content of negative electrodes, and further reduce the active material content of positive electrodes, PA1 (a) Accept 9 to 18 kw of power over a 6 second period. PA1 (2) Rest 20 seconds. PA1 (3) Deliver 12 kw of discharge power for 3 seconds. Rest 21 seconds. PA1 (4) Over a 3 second period deliver power at a rate that will suffice to maintain constancy of state of charge over a large number of charge-discharge cycles. PA1 (5) Rest 20 seconds. PA1 (6) Next repeat the above cycle and thereafter continue to repeat, with the state of charge that would be maintained from cycle to cycle being chosen such that, on occasion, the battery, when called on to do so, would be able to deliver two 18 kw, plus three 12 kw, power pulses each of 3 seconds duration, absent any charging between discharges. PA1 (7) Anticipated weight 80 pounds including no more than 11/2 lbs. of cadmium. PA1 (8) Anticipated life in excess of 100,000 cycles. FNT *Numbers in parenthesis refer to numbered items in table of reference. PA1 (a) employment of other than usual electrode pore loadings, PA1 (b) best electrode and cell configuration, PA1 (c) devising a charge control scheme that would avoid cell damage due to gas evolution as a result of overcharge, yet that would also provide so that requirements (2), (6) and (8) above could be met. PA1 "precharge is set by electronically discharging at a 0.075 ampere rate (power discharging) the full negative group against the cell container" PA1 (a) provide for avoidance of cell damage due to too rapid or too long continued regenerative braking, and, PA1 (b) at the same time operate to minimize delay in restoration of battery discharge capability following deep discharge, PA1 (a) supply power for accelerating the vehicle, and PA1 (b) accept charge when the vehicle is being braked,
Ford Motor issued to Battery Development Corp. of New York, N. Y., for which firm the writer's studies of nickel-cadmium batteries were being conducted, a Request for Quotation (6)* covering production of a pair of 24 volt low energy, but high power density nickel-cadmium batteries that would utilize a minimum amount of cell active materials, and that would be intended to meet performance requirements as below:
On receipt of a copy of the Request for Quotation, the writer initiated laboratory production of two inch by two inch positive and negative electrodes that were low pore loaded to varying extents, and next proceeded to carry out tests with use of the test cycle called for, the intention being to verify what was feasible and give consideration to possibly patentable concepts prior to quoting.
The writer perceived three possible areas of this type, as below,
As matters turned out the writer, before long, concluded from test results, that low, though not as low as called for, negative electrode pore loading, would be feasible, and also that a new cell configuration could prove advantageous.
But there remained the problem of how to control charge processes in a way that would take account of the fact that, under conditions of use, the operators of hybrid drive type vehicles will fail to apply pressure to the vehicle's accelerator and brake pedals in any consistent or predictable manner, but, rather, will perform these functions in variable ways, dependent on the exigencies of traffic, in which connection it is worth noting that the Ford Scientific Laboratory personnel that had charge of the previously referred to dynamometer tests, found it necessary, in those tests, to provide to manually terminate charging in response to visual evidence of rapid evolution of gas within the cells (7).
When it comes to techniques for fast charging nickel-cadmium batteries, published material that has come to the writer's attention has made reference to "dump-timed" charging, in which a battery is first fully discharged and thereafter charged for a predetermined time at a constant rate, which technique, per reference 8, has been used to "charge sealed nickel-cadmium cells up to 40 percent of rated capacity in about 3 minutes", while reference 9 cites ability to charge fully discharged cells of sealed nickel-cadmium type to 1/4 of rated capacity in one minute.
However, reference 8 states also that "dump timed" charging can be used only with single cells "because dump discharging a multicell unit would permantly damage the battery if individual cells were driven into reversal".
As a solution to the problem of avoidance of damage of one or more of a group of series connected nickel cadmium type cells due to cell reversal, while, in one approach damage, can be avoided by automatically terminating battery discharge when voltage falls below a sufficiently high value that depends on cell current, where, as in the case of hybrid drives for vehicles, capability for deep discharge is especially important, the writer proposes to employ, in addition, a protective technique developed by NASA's Marshall Space Flight Center in accordance with which a multiple output type of dc to dc converter is utilized as a way to constantly provide for cell equalization under conditions of battery use (10).
When it comes to operation of hybrid drive vehicles in traffic, if, as can sometimes occur, traffic moves very slowly, for a long enough period, it is inevitable that the state of charge of the battery of an engine-electric drive system using the Ford B strategy will be so much reduced that it will be necessary to provide for the operation of the vehicle's drive system to revert to conventional drive in which the engine can operate when the vehicle is at rest, and would be caused to do so long enough, and at sufficient speed, to effect rapid recharge of the battery.
However, from the standpoint of fuel economy it would be undesirable that reversion take place other than infrequently, which being the case, the desirability of inclusion of a discharge capability requirement along the lines of item 6 of the Ford Motor Request for Quotation is indicated.
On the other hand, the concept contained in the Ford Request for Quotation, to the effect that, per item 4, it would be planned to accept, in 6 seconds, more energy via regenerative braking than would be given up by the battery during a 12 kw discharge lasting 3 seconds, while offering advantages from a fuel saving standpoint, in practice has the effect of requiring use of a larger battery than would be needed merely to meet discharge requirements, and for this reason deserves to be viewed as of questionable merit.
On this score the writer concluded that, in the interest of holding battery weight low, it would be desirable to plan testing on the basis of employment of a battery primarily sized to meet discharge requirements, and this approach to design of an engine-electric hybrid drive system, is adhered to in the preferred embodiment portion of the present application.
When charging of either a nickel-cadmium or lead-acid battery first begins after entire discharge, gas evolution does not at once occur, provided that the magnitude of charge current is sufficiently restricted. However, when, during a charge process, state of charge is allowed to exceed a value that depends on battery type, and may also depend on cell internal temperature, gassing occurs even if rate of charge is small, while, as state of charge progressively increases, need to restrict charge rate increases, if undesirably rapid gas evolution, and reduction of discharge-recharge efficiency is to be avoided.
In employment of a nickel-cadmium battery in hybrid drive systems for vehicles, because of fast discharge-recharge requirements, employment of sealed cells, either of the widely sold cylindrical type or the rectangular plate type employed in space vehicles, in both of which only enough electrolyte is employed to dampen the electrode separators, is viewed by the writer as impracticable and therefore, employment of flooded cells, or cells that would be nearly fully flooded has been planned.
However, in the case of such cells, in order to avoid, or, at any rate, hold to a minimum, need to periodically add water to cells, and also avoid undesirable change in the relative state of charge of the cell's positive and negative electrodes, it is desirable to devise and provide to employ a type of charge control system that holds rate of gas evolution within the cell during charge low enough so that, over a period of time, whatever gas has been generated during charging and has found its way into the space above the electrolyte is reabsorbed into it, where, if it is oxygen, it can diffuse to and react with the cadmium of the negative electrodes, and if it is hydrogen, can diffuse to and react, even if only slowly, with the active material of the positive electrodes, and do so notwithstanding the fact that unlike what applies in the case of the sealed cells of commerce, and those used in space craft, access of gases generated within the cells to the electrodes is largely diminished due to the fact that the electrodes are either fully or nearly fully submerged, and the further fact, that, in use of a small battery, in hybrid drive that employs regenerative charging very rapid charging up to a predetermined state of partial charge is called for.
Though the writer is unaware of any prior art in this area the writer's tests have shown that devising a charge control system that will meet these requirements is feasible, provided that, as in the case of flooded porous plate type nickel-cadmium cells, of aircraft type, the cells are provided with pressure relief type vents which operate to hermetically seal the cells so long as cell internal pressure holds below a predetermined value, yet allow discharge of gas in case of control system malfunction, which, in turn implies that aircraft type nickel-cadmium cells can be made use of in hybrid drive systems for road vehicles, with normal absence of discharge to atmosphere of internally generated gases, though, as previously noted, it has appeared that there can also be advantages in employing low pore loadings and a new type of cell configuration.
When it comes to spacecraft type sealed cells it is well known in the art pertaining to their production and use that it is important to ensure that when their electrodes are assembled into them the extent of precharge of the negative plates, by which is meant the extent of charge of those plates when the positive plates have been fully discharged, expressed as a percentage of positive plate charge capability, is suitably controlled (11), though this requirement has been examined only for the case of cells that, in use, would normally be largely or fully charged, and there appears to be no evidence of published information that applies in the hybrid drive case, or at any rate to a hybrid drive of the Ford B strategy type in which maximum state of battery charge is held low.
However, the writer's tests well demonstrated that it is necessary to see to it, that, under typical hydrid drive vehicle use conditions the relative states of charge of the positive and negative electrodes is suitably controlled, and, insofar as possible, stabilized.
In one approach it would be possible to arrive at a solution to this problem in the manner set forth in the last paragraph of reference (11), but this would introduce prohibitive cell cost in a hybrid drive application, and would fail to provide for cell stabilization.
In this area it is an aspect of the present invention that it discloses a relatively simple way to achieve desired control of state of charge of electrodes in a largely stabilized form, which comprises slowly charging cells, while venting internally generated gas, until both the positive and negative electrodes are fully charged, and next vacuumizing and filling with nitrogen.
As to prior art the writer has noted that on page E-59 of reference (12) mention is made of effecting nickel-cadmium cell state of charge adjustment "by a hydrogen or oxygen venting technique established by the seller" (ie. by the cell producer), but further details are not supplied. Also, on page 3-3 of reference (12) the statements are made
a very slow process that, in any case, would not be applicable where a plastic cell case was employed.
Also another statement on the same page reads to the effect that after vacuum tests "The cells are back filled with an oxygen-helium mixture" a mixture of gases that clearly is not inert.
Accordingly as far as the writer knows fully charging, vacuumizing and then filling with an inert gas represents a heretofore not used, and, as it would appear, non-obvious technique.
Also, while the problem of providing so that, in regenerative charging of batteries of hybrid drive systems for road vehicles, enough motor voltage can be generated to cause power to flow into the battery has been approached in a variety of ways, including automatic battery reconnection from a series to a parallel connection of two equal groups of cells (15, 16), use of a chopper type voltage booster (13, 14), and control of generator field current (13, 15, 16, 17), in the case of published materials that have dealt with regenerative braking that have so far come to the writer's attention, none have given consideration to automatic controls that would,
despite the fact that employment of a control system that will serve these purposes appears to be essential to ability to effectively employ, in a road vehicle equipped with an angine-electric type hybrid drive system, a propulsion battery that is both low in weight, and that will exhibit favorable performance and long discharge-recharge cycle life under service use conditions.
The present invention is directed to providing an answer to problems in these areas.
While by far the bulk of the writer's experimental studies have been confined to nickel-cadmium batteries, it appears that what the present disclosure teaches will prove to have useful application to hybrid drive systems for vehicles that would use a battery of lead-acid, nickel-iron, and nickel-zinc types, and it is further viewed as probable that the same would prove to be true in the case of any other types of batteries that would prove to be adaptable to use as fast discharge-recharge components of hybrid type vehicle drive systems.